U.S. patent application number 12/384825 was filed with the patent office on 2010-10-14 for solenoid drive method that conserves power.
This patent application is currently assigned to Pertech Resources, Inc.. Invention is credited to Jeff Maurice Wendt.
Application Number | 20100259861 12/384825 |
Document ID | / |
Family ID | 42934191 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100259861 |
Kind Code |
A1 |
Wendt; Jeff Maurice |
October 14, 2010 |
Solenoid drive method that conserves power
Abstract
A drive method for an impact-printhead solenoid is provided that
improves power efficiency with an extremely simple circuit
configuration and no sensors. Consisting only of a power-FET
(Field-Effect Transistor) and PWM (pulse-width-modulation) signals
from a printer-controller, this system, using a novel PWM
frequency-optimization technique, reduces printhead power usage by
as much as 13%.
Inventors: |
Wendt; Jeff Maurice;
(Lander, WY) |
Correspondence
Address: |
Douglas P. Etsinger PE,;Pertech Resources, Inc.
860 College View Drive
Riverton
WY
82501
US
|
Assignee: |
Pertech Resources, Inc.
Riverton
WY
|
Family ID: |
42934191 |
Appl. No.: |
12/384825 |
Filed: |
April 10, 2009 |
Current U.S.
Class: |
361/160 ;
347/110 |
Current CPC
Class: |
H01H 47/32 20130101 |
Class at
Publication: |
361/160 ;
347/110 |
International
Class: |
H01H 47/00 20060101
H01H047/00; B41J 2/00 20060101 B41J002/00 |
Claims
1. An energy-saving solenoid-drive circuit and method comprising: a
power supply, a switch means, a solenoid, and a controller to
repetitively energize the circuit and solenoid.
2. The energy-saving solenoid-drive circuit of claim 1, where the
switch means is a power-FET.
3. The energy-saving solenoid-drive circuit of claim 1, where the
switch means, and solenoid are one of a multiplicity of print-wire
driver-circuits and solenoids, as in an impact printhead.
4. The energy-saving solenoid-drive circuit of claim 1, where the
controller is a microcontroller and part of an impact printer's
main control system.
5. The energy-saving solenoid-drive circuit and method of claim 1,
where the controller provides pulse-width modulated signals to the
circuit and solenoid, the period and frequency of which are
pre-determined mathematically, then, refined empirically, through
modeling of the terminal parameters of the semiconductors and
inductor in the solenoid drive circuit, such that flyback energy
from the on and off conditions of the solenoid, is reduced
significantly.
Description
CROSS-REFERENCE TO RELATED INVENTIONS
[0001] Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
REFERENCE TO SEQUENCE LISTING, OR A COMPUTER PROGRAM LISTING
COMPACT DISC APPENDIX
[0003] Not Applicable
BACKGROUND OF THE INVENTION
[0004] This invention relates to power-saving techniques applied to
electronic solenoid-drive circuits, and specifically relates to
power saving in an impact printer that uses solenoid-driven
print-wires under the control of a microcontroller.
[0005] Solenoids that convert electrical energy to mechanical
movement are well known and can be found in hundreds of varieties.
Relays, water-valves, automobile starter solenoids are just a few
examples. Also numerous are the means that operate the solenoids.
Electromechanical relays were state-of-the-art before solid-state
devices entered the scene with lower cost and more efficiency. In
recent decades, the electronic circuits driving the solenoids have
become more and more sophisticated. The use of microcontrollers and
fast-switching transistors have improved even more the precision
and efficiency of solenoid actuators.
[0006] In the field of impact printheads and printers, it is common
to provide a number of identical print wire actuators, commonly in
a 9 or 24 wire dot matrix, all driven under microcontroller
control. It is well known that the print wires were accelerated
into an inked-ribbon, which then placed dots arranged as characters
and numbers onto a printed page. Two predominant types of actuators
exist: [0007] a. A magnetically driven hammer or clapper,
comprising the frame or armature of a solenoid, strikes and
accelerates a print-wire, and, [0008] b. A magnetically driven
plunger inside the core of a solenoid, attached to a print wire,
accelerates a print wire.
[0009] In either type, the electrical circuits were similar, and,
efforts to conserve energy were very similarly applied, whether the
circuit was organized as a constant-current type or as a constant
voltage type. The former type offered the best control but was also
the most expensive to implement. PWM techniques improved the
designs even more, offering a constant-current solution without the
expense, especially enabling a more conservative use of energy in
the printhead, which is the largest consumer of energy in an impact
printer. In fact, the heat created because of wasted energy in an
impact printhead and the drive circuits has forced limits on the
print-head's print speed. The limits are needed to prevent
component failure. Earlier impact printers were forced to run
slower or were forced to go into "slowdown" modes when temperatures
reached upper limits. Consequently, extra sensors were required to
monitor temperatures or print-speeds. This imposes an undesirable
performance limitation on a printing system that is often marketed
on throughput. Additionally, because of other earth-global issues,
energy conservation in product design has become paramount. As a
result, a number of energy-conserving techniques exist in the prior
art. A number of patents and other documents cite the recycling of
flyback energy, created when a solenoid is turned off, back into
the power supply, or, to a storage device for reuse. However, there
is untapped flyback energy to be saved in another area, which is
the focus of this invention.
BRIEF SUMMARY OF THE INVENTION
[0010] The object of this invention is to present an additional and
novel method, without extra electronic or mechanical components, to
significantly reduce wasted energy in a solenoid actuator system.
This method can be applied in any application where a
solenoid-operated device, using PWM techniques to control current,
is used. The preferred embodiment, a printer with an impact
dot-matrix printhead, is summarized and described in detail. This
invention improves on the pulse-width current control by optimizing
it. There is no claim or discussion in this invention regarding any
processing of or redirection of the flyback energy pulse appearing
at 1b, FIG. 5, or, at 1d, FIG. 6.
[0011] FIG. 1 comprises the few hardware components necessary to
operate one of "n" solenoid-actuated print wires in a dot-matrix
printhead. A frequency/duty-cycle specific PWM signal, FIG. 4, is
applied at 4, FIG. 1. The frequency and duty-cycle are described
mathematically in the following paragraphs, and refined empirically
at the product design level, after the selection of circuit
components, namely, the solenoid drive FET 8. The selected FET's
data-sheet reveals its gate capacitance 8a. This value is then used
to set the PWM signal's on and off times, and this value should be
fine-tuned for real world applications. This gate capacitance, in
conjunction with the inductance of the print wire actuator coil and
the finite resistance inherent to the circuit, creates a
configuration which may best be modeled with 2.sup.nd-order
differential equations, FIG. 8, yielding an exponential, sinusoidal
damping effect on the current flowing through the solenoid.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0012] FIG. 1 Circuit Diagram of the preferred embodiment of the
solenoid drive.
[0013] FIG. 2 Small-Signal, Equivalent Circuit Model of the
preferred embodiment.
[0014] FIG. 3 General equation for current flow through a MOSFET
device.
[0015] FIG. 4 PWM waveform detail and truth table of AND-gate
device.
[0016] FIG. 5 Illustration of non-optimized waveforms at numbered
circuit nodes.
[0017] FIG. 6 Illustration of optimized waveforms at numbered
circuit nodes.
[0018] FIG. 7 Exaggerated View of Ip and exponential decay
overlay.
[0019] FIG. 8 Solution Equations
DETAILED DESCRIPTION OF THE INVENTION
[0020] FIG. 1 is the schematic diagram of the preferred embodiment,
and shows only the components required of this invention. FIG. 2 is
the small-signal equivalent-circuit model of FIG. 1 that sets up
the mathematical solution. Circuit variations can occur without
deviating from the spirit of the invention. For example, other
components and signals, some described previously as prior art, can
be added to further enhance the power efficiency or adapt to other
applications. This narrative will apply to the preferred
embodiment, a dot-matrix impact printer. The circuit has a power
input 10, solenoid 9, N-channel power-MOSFET 8, and
printer-controller 11. Only the required parts of the
printer-controller are shown, such as the logic AND-gate 6, FET
gate-current limiting resistor 7, and input signals Vwire 3, Vpwm
4. Connector 9 represents one of multiple circuit connections to a
dot-matrix printhead, which often has 9, 12, 24, or more, duplicate
solenoid circuits.
[0021] Power input 10, often 24 vdc, but not a critical voltage to
this invention, provides the potential to operate the solenoid. The
printer-controller operates at 3.3 vdc in this embodiment, but this
value is not critical to this invention.
[0022] Noting FIG. 1, the printer controller 11 controls
energization of the circuit. One of "n" print wires is selected by
placing a logic1 signal at 3 along with a logic1 signal at 4. The
AND gate 6 turns on and off in conformance with its truth table
FIG. 4, and presents its signal 2 at the gate of FET 8 through
resistor 7. Those familiar with the art of digital systems will
readily see that a logic1 is +3.3 vdc in this embodiment, and
logic0 is zero volts. A positive gate voltage at 2 will turn on the
FET 8 causing current Ip 5 to flow from the power supply 10,
through solenoid 9, and through FET 8 to ground. Noting FIG. 5, the
signal 4 is steady in the on state until time 13, where it changes
to a pulse-width-modulated (PWM) signal. Ideally, this PWM waveform
will be transmitted from the AND-gate 6 to the FET 8 in real time.
However, due to the capacitance of the FET gate as well as the
capacitance of the AND gate itself, Vg FIG. 1 will follow the
established equations for voltage and capacitance. See FIG. 2.
[0023] FIG. 5 illustrates the actual voltage levels as they appear
un-optimized at nodes 1,2,3,4, and the un-optimized print wire
solenoid current Ip at node 5. 1a shows lost power as flyback
voltage, typically dissipated as heat somewhere in the circuit.
[0024] FIG. 6 illustrates the waveforms after optimization, which
are the subject of this invention, and described as follows:
Viewing FIG. 5 at time 12, the circuit becomes energized. Logic1 at
both nodes 3 and 4 cause the level at node 2 to also rise to a
logic1 level. As a result, FET 8 then turns on, effecting a very
low resistance between node 1 and ground. Again viewing FIG. 5, as
the circuit is full-on, current 5 rises quickly in the solenoid, in
accordance with equation 1 FIG. 3 and equation 3b FIG. 8. As
detailed in the prior art, the print hammer (clapper), being moved
by the rising magnetic field, is accelerating the print-wire. At
approximately time 13, the solenoid has reached saturation and max.
magnetic field, and a PWM signal 4 is applied to control Ip from
rising higher, effecting a "constant current" between time 13 and
time 14. Also, during the same period from time 13 to time 14, the
FET drain-voltage Vd at waveform 1a, FIG. 5, appears. This is
solenoid flyback energy appearing across the FET at PWM frequency.
Measured waveforms at 1a and 5, FIG. 5, confirm empirically what is
already well-known, that, driver-circuits that employ
constant-current drives, or use PWM to approximate constant-current
drives, will cause a resultant power dissipation to move from the
solenoid to the FET and manifest itself as heat, and, obviously,
wasted energy. The thermal mathematics will not be addressed,
here.
[0025] It will be shown that the mathematics, verified with
empirical observations, prove that the PWM signal can be adjusted
to a point where the circuit still maintains a constant average Ip,
yet, eliminates the flyback energy from dissipating across the FET
8 at 1a, FIG. 5. The period and duty-cycle of this PWM signal are
such that the net effect on current Ip is that it becomes an
exponentially decaying sinusoid, seeking a steady-state optimal
value, in this case 1.6 amperes, at 16, FIG. 6. This is also shown
in FIG. 7. as an exaggerated view of Ip with its decaying sinusoid
shape, described by equation 5, FIG. 8, based in part on equations
1 through 4.
[0026] A short discussion of semiconductor specifications is
necessary to complete the described technique: All semiconductor
devices have specified in their data-sheets parameters of voltage,
current, capacitance, frequency limits, and numerous operating
limits, all of which enable the designer to accomplish a circuit
that works to his needs. Reference FIG. 2, the small-signal
equivalent circuit model. In this invention, the designer, having
selected a drive-transistor, in this case a particular MOSFET, uses
its gate capacitance, by applying a high frequency PWM signal, to
limit the device's turn-on and turn-off, therefore producing a
smoother waveform. Specifically, the gate-capacitance in
combination with the inductance of the solenoid-coil combined with
the power-FET's real world resistance establishes a physical
reality which can be modeled by second order differential equations
in FIG. 8, yielding an exponentially damped sinusoid. Compare
waveform 5, FIG. 5 to waveform 5a, FIG. 6.
[0027] When the solenoid has exhausted its ability to effect
additional acceleration of the wire, the solenoid is shut off at
time 14. This shutoff at time 14 is well described in prior art and
is not part of this description. The large pulse 1b and 1d
appearing at time 14 to time 15 is the flyback energy created from
the magnetic field collapse during solenoid shutoff. As indicated,
the recovery and reuse of this particular flyback energy pulse is
also well described in prior art and is not part of this
description.
* * * * *